User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Preferences > E-mail Options
User
HKCU\software\policies\microsoft\office\16.0\outlook\options\mail
This policy setting controls whether Outlook displays pictures in email headers for senders of email messages and meeting requests. If you enable or do not configure this policy setting Outlook displays pictures for senders if they are available. If you disable this policy setting, pictures of senders will not be displayed.